Forces & Stresses
What Are Forces & Stresses?
Every product experiences forces during use. A shelf bears weight, a bridge resists wind, a chair withstands the sitter's mass. Understanding the five main types of force helps designers choose materials and shapes that will not fail. You need to identify, explain and apply these forces to real products across all material categories.
The five types of force and stress are: tension (pulling apart), compression (pushing together), bending (a combination of tension and compression), torsion (twisting), and shear (sliding forces in opposite directions).
The Five Forces Explained
Tension occurs when a material is pulled from both ends. Think of a washing line holding wet clothes, or the steel cables on a suspension bridge. Materials under tension are being stretched.
Compression is the opposite — a squashing force. The legs of a table experience compression from the weight of items placed on top. Concrete is strong in compression, which is why it is used for building foundations.
Bending combines tension and compression. When you stand on a plank supported at both ends, the top surface is compressed and the bottom surface is in tension. If the material cannot resist both, it snaps.
Torsion is a twisting force. A screwdriver shaft experiences torsion when you turn it. The key to resisting torsion is using a solid cross-section or a tube — hollow circular sections are especially efficient.
Shear acts when two forces push in opposite directions along parallel planes. Scissors work by applying shear force to paper. A rivet holding two metal sheets together resists shear as the sheets try to slide apart.
Forces Across Material Categories
Timber: Good in compression along the grain but weaker across it. Plywood resists forces in multiple directions because its layers are glued with alternating grain.
Metals: Steel has excellent tensile strength, making it ideal for cables and structural beams. Cast iron is strong in compression but brittle under tension.
Polymers: Thermoplastics like polypropylene flex rather than snap, absorbing bending forces. Rigid polymers like acrylic can shatter under impact.
Textiles: Woven fabrics resist tension along the warp and weft. Knitted fabrics stretch more, making them better for products that flex.
Papers & boards: Corrugated card resists compression because the fluted inner layer acts as a series of small arches.
Reinforcing & Stiffening Materials
Materials can be modified to better resist forces. You need to know specific techniques and explain why they work.
Common reinforcement methods include: lamination (gluing layers together), folding (creating flanges or channels), webbing (adding ribs or gussets), and fabric interfacing (bonding stiffening material to textiles).
Lamination: Gluing thin layers of material together increases strength. Plywood is a classic example — odd numbers of veneers with alternating grain direction resist warping and cracking. Laminated steel is used in transformer cores.
Bending and folding: A flat sheet of metal is weak in bending, but fold it into an L-shape or a box section and it becomes far stiffer. Corrugated card uses the same principle — the wave-shaped flute resists compression.
Webbing and gussets: Triangular gusset plates at the corners of a shelf bracket spread the load and prevent the joint from collapsing. Internal ribs in plastic mouldings add stiffness without much extra weight.
Fabric interfacing: Iron-on interfacing is bonded to textile panels (e.g. collar and cuffs) to add structure and resist deformation.
Changing Magnitude & Direction of Forces
Designers can redirect or reduce forces using structural shapes. An arch converts a downward load into compression along its curve, pushing outward into its supports. A triangulated frame (like a roof truss) distributes loads efficiently because triangles cannot be deformed without changing the length of a side. I-beams concentrate material at the top and bottom flanges where bending stress is greatest, saving weight in the middle.